Antioxidants of bisphenolic polymers

ABSTRACT

For incorporation in normally oxidizable polymers, polymeric antioxidants characterized by repeating hindered bisphenolic units, the average number of said units per polymeric chain in said composition being from about 2 to 10, preferably about 2 to 7, most preferably about 2 to 4. The antioxidants are prepared by oxidative coupling of appropriate bisphenols, as by reaction with a ferricyanide coupling agent. Polymer-antioxidant combinations formed according to the invention have excellent high temperature oxidative stability, high resistance to polymer damage by massive doses of ionizing radiation and the antioxidant does not release significant amounts of condensable species when subjected simultaneously to conditions of high vacuum and elevated temperatures.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation-in-part of my copending application Ser. No. 150,831 filed June 7, 1971, now abandoned which was in turn a continuation-in-part of my application Ser. No. 660,198, filed Aug. 14, 1967, now abandoned.

BACKGROUND OF THE INVENTION

Olefin polymers are particularly subject to severe deterioration from the oxidative action of air at elevated temperatures. They are also vulnerable to degradation by the action of ionizing radiation when large amounts of such radiation are deposited into the polymer, e.g., amounts of 5 × 10¹⁰ ergs per gram and higher. Oxidative deterioration at elevated temperatures is manifested by surface crazing, then cracking and ultimately a complete embrittlement.

It is known that small amounts of some antioxidants, e.g., 4,4'-thiobis(6-5-butyl-m-cresol), can be added to polyolefins and more particularly to polyethylenes to prevent or impede these degradative effects without the contribution of color to the polymer. However, such conventional antioxidants are not effective under conditions of high vacuum and/or high temperatures since with time the antioxidant volatilizes away from the substance into which it was incorporated.

Until recently, the volatility of such antioxidants in a crosslinked polyolefin composition was undesirable primarily because of the reduction in oxidative stability which occurred as the antioxidant volatilized away.

The use of crosslinked polyolefin materials in space vehicles has brought to light a still more important disadvantage of these antioxidant materials. When used in a space vehicle, a crosslinked polyolefin material, employed for example in the form of electrical wire installation on solar panel wiring or in front of tubing, may be exposed to solar radiation in the hard vacuum of space such that the polyolefin may be heated to temperatures as high as 125° to 150° C. Under such conditions, most of the prior art antioxidants, such as those previously mentioned, will be volatilized away. Since optical systems in the space vehicle, shielded to solar radiation, may be at a relatively low temperature, perhaps 10° C., the gas formed from the antioxidant will condense on the optical system thereby adversely effecting its transmitive and reflective optical characteristics. Furthermore, the heated polyolefin with its antioxidant-radiation protectant volatilized away is now vulnerable to radiolytic degradation such as that which may be caused by energetic solar protons.

It is also known that certain polymeric involatile materials may be used as antioxidants. Thus, certain phenolformaldehyde polymers may be used as antioxidants. Also, bisphenols and triphenols such as those described in U.S. Pat. No. 3,247,262 may be used to protect against oxidative degradation. The bis and triphenols are still volatile, however. The phenol-formaldehyde resins, which have lower volatility, have disadvantages including thermosetting characteristics of the phenol-formaldehyde antioxidant, choice of precursors on the basis of synthetic convenience rather than antioxidant capability and a pronounced tendency to impart coloration to polymers to which they are added.

U.S. Pat. No. 3,133,899 to Kwiatek proposes the copper chloride-catalyzed oxidative polymerization of a thiobisphenol to form polymeric ethers of formula ##SPC1##

the thiobisphenols employed having the formula ##SPC2##

wherein at least one R on each phenolic group is alkyl, the other being alkyl or hydrogen, alkyl in every case preferably having 1 to 3 carbon atoms. Should those R groups be of more than 3 carbon atoms, the work of Hay et al. in J.A.C.S. 81,6335 (1959) with oxidative coupling of 2,6-disubstituted phenols would suggest that the Kwiatek procedure would yield diphenoquinones as the sole product. Such materials form undesirable color bodies when blended in olefinic polymers and the like.

Braus et al., in U.S. Pat. No. 3,462,375 disclose antioxidant compositions of formula ##SPC3##

but the values assigned Z₁₋₂, Y₁₋₃ and M₁₋₃ make clear that it is a phenol-substituted benzene compound rather than a bisphenolic polymer which is proposed. Again, in U.S. Pat. Nos. 3,377,333 and 3,044,960, respectively, to Ciesielski et al. and Norway et al. alkyl-substituted thiophenolic polymers are disclosed whose repeating units are, e.g., ##SPC4##

Such polymers are distinguished from the bisphenolic polymers of the present invention by, inter alia, the characteristic lack in the former of direct carbon-carbon bonding between the phenolic rings of adjoining thiobisphenolic repeating units.

SUMMARY OF THE INVENTION

According to this invention, there are provided antioxidants and antioxidant-containing polymer compositions, the antioxidants having the formula ##STR1## wherein n is from about 2 to 12 and wherein for each repeat unit, Z and X are independently selected from the group consisting of ##SPC5##

R₂ and R₃ being independently hydrogen or alkyl, cycloalkyl, aryl or alkaryl groups having from 1 to 8 carbon atoms, R₄ being alkyl of 4 to 8 carbon atoms, preferably tertiary alkyl and Y is selected from the group consisting of sulfur methylene and butylidene. It has been found that the controlled oxidative coupling reaction of certain bisphenolic compounds will produce a non-volatile compound which is surprisingly effective as an antioxidant. The polymeric antioxidants of the present invention are generally colorless or only very slightly colored and thus may be used at relatively high levels, i.e., 1% in polymers such as polyolefins without causing undue coloration in the finished fabricated article. The antioxidants of the invention are resistant to the effects of high temperature and high vacuum and are capable of imparting radiation resistance to polymeric compounds.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The low molecular weight phenolic compounds suitable as precursors in this invention have the following formulae: ##SPC6##

wherein R₁ is hydrogen; R₂ and R₃ are independently selected from the group consisting of hydrogen or alkyl, cycloalkyl, aryl or alkaryl groups having from 1 to 8 carbon atoms, R₄ is an alkyl group having from 4 to 8 carbon atoms and Y is selected from the group consisting of sulfur, methylene or butylidene. R₄ is preferably tertiary alkyl, i.e., at least one carbon atom is substituted only by carbon. Phenolic moieties whose valence bonds to sulfur are para to hydroxyl, such as compound (2) above are preferred from the standpoint of avoiding coloration of polymers in which the antioxidants of the invention are employed.

The low molecular weight bisphenolic compounds suitable for use as precursors in this invention may be selected from a wide variety of available phenolic compounds which have antioxidant properties, e.g., 4,4' methylene bis(6-t-butyl-m-cresol); 4,4' thiobis (6-t-butyl-m-cresol); 4,4' butylidene bis(6-t-butyl-m-cresol); 2,2'-methylene bis (6-t-butyl-m-cresol); 2,2' thiobis (6-t-butyl-m-cresol); 2,2'-butylidene-bis(6-t-butyl-m-cresol). These and other bisphenolic precursors suitable in this invention may be conventionally prepared by, e.g., reaction of one or more phenolic compounds with sulfur mono- or dichloride; by the reaction of olefin and a suitable thiobisphenol in the presence of an alkylation catalyst, etc.

The precursor used to produce the antioxidants of the present invention must have available at least one unsubstituted ortho or para position relative to the phenolic hydroxyl groups. For example, in 4,4'-thiobis(6-t-butyl-m-cresol) there are two ortho positions unsubstituted.

Normally, oxidative coupling yields a mixture of molecular species of varying length (e.g., n ranges from 2 to about 12), n average for the mixture being from about 2 to about 10, preferably from about 2 to about 7, and most preferably from about 2 to about 4.

The antioxidants of the present invention are useful in polymers generally and particularly olefin polymers and copolymers such as polyethylene, polypropylene, polyisoprene, ethylene-propylene copolymers, acrylic polymers such as polyethyl acrylate and other polymers such as polyamides, Teflon, etc., which are known to be stabilized by hindered phenols. The antioxidant can be incorporated in effective amounts into the polymer by conventional methods such as hot milling, mixing in a Banbury mixer, extrusion, etc. Normally, from about 1 to about 10 percent by weight of antioxidant is employed, based upon the weight of the normally oxidizable polymer.

The final polymeric systems containing these polymeric stabilizers are particularly suitable where it is desired to crosslink by the use of ionizing radiation because these stabilizers do not appreciably inhibit crosslinking as compared with the low molecular weight precursors and are more resistant to radiation damage.

The coupling reactions suitable for use in this invention include oxidative coupling of the bisphenols by oxidizing agents such as K₃ Fe(CN)₆, chromic acid, organic peroxides, manganese dioxide, potassium permanganate, potassium dichromate and electrolytic oxidation. Coupling agents which do not form molecular oxygen upon decomposition are employed to avoid formation of undesirable by-products. Coupling may take place in situ although it is preferred to perform the coupling reaction under conditions such that the structure and nature of the resulting antioxidant can be controlled to prevent undesirable side reactions.

It has been found that the antioxidant polymer of this invention is particularly effective if it is formed using a ferricyanide coupling agent, e.g., K₃ Fe(CN)₆. The increased effectiveness obtained by this method may be due to the fact that undesirable by-products are substantially reduced. Thus, other coupling systems may produce by-products which detract from the effectiveness of the antioxidant. In any event, the antioxidants prepared with K₃ Fe(CH)₆ do exhibit increased effectiveness when compared with antioxidants of this invention prepared from the same precursors with other oxidizing agents.

Any suitable ferricyanide may be used in the coupling reaction, e.g., K₃ Fe(CN)₆, Na₃ (CN)₆, etc. The molar ratios of ferricyanide to bisphenol may vary from about 0.5 to 2 with the preferred ratio being about 1.33 to 1.

Increasing the ratio of ferricyanide to bisphenol will increase the molecular weight of the antioxidant. At a ratio of 1.33 to 1, the resulting polymeric antioxidant will contain an average of about 3 bisphenol units. The molecular weight can also be controlled by controlling the reaction period, a shorter reaction time resulting in lower molecular weights.

The ferricyanide may be added to the bisphenol as a water solution. Addition may be dropwise or the entire water solution may be added at once to the bisphenol dissolved in an organic solvent.

It is important to maintain the pH of the reaction solution near 7. Thus, the pH may vary from about 5 to 9 without any substantial adverse effect. In order to maintain the pH at the desired level, an acid acceptor or buffer such as sodium carbonate or bicarbonate can be used. In general, any acid acceptor which will maintain the pH near 7 may be used.

The reaction temperature should be maintained at as low a level as possible. Thus the temperature should be maintained below about 80° C. and preferably below about 50° C. If the temperature is allowed to exceed 80° C., discoloration of the polymer and decreased antioxidant effectiveness may result.

The oxidative coupling using an organic peroxide can be accomplished either in solution or in situ by adding the peroxide to a polymer containing an antioxidant precursor. Suitable peroxides include t-butyl perbenzoate, dicumyl peroxide, benzoyl peroxide, 2,5-dimethyl-2,5-bis(t-butyl-peroxy)hexane, 2,5-dimethyl-2,5-di(t-butyl-peroxy)hexyne-3, t-butyl-peroxy-isopropyl carbonate, etc.

Suitable solvents in which the coupling with organic peroxide can be carried out include cyclohexane, benzene, toluene, xylene, chlorobenzene, etc. In general, the preferred solvent with peroxides is xyxlene, because of its lower vapor pressure at the convenient decomposition temperature of the peroxide. Benzene is preferred when a ferricyanide coupling agent is used.

The molar ratio of peroxide to the antioxidant precursor should be maintained between a lower limit of about 0.5 and an upper limit of about 2 to 1 with a ratio of about 0.9 to 1 being preferred.

The present invention is further illustrated by the following examples in which parts are by weight unless otherwise indicated.

The effectiveness of the antioxidant was measured by temperature programmed differential thermal analysis in oxygen. In this technique, which has been described by V. E. Althouse (Am. Chem. Soc., Div. Polymer Chem. Preprints 4, (1), 256 (1963)), the temperature of a polymer sample containing a certain percentage of the antioxidant is increased at a uniform rate (in this case, 61/4° per minute) with a constant flow of oxygen, until an autocatalytic oxidation (accompanied by an exotherm) occurs. The temperature at which the oxidation exotherm appears has been shown to be proportional to the logarithm of the concentration of an antioxidant and to indicate to a degree its effectiveness as an antioxidant.

The temperature at which an oxidative exotherm occurred (hereafter known as the DTA temperature) was measured by the above procedure on the polymer samples as prepared, and also after the samples has been maintained at 200° C. for 66 hours in a slow stream of argon. It is found that lower molecular weight antioxidants, such as 4,4'-thiobis (6-t-butyl-m-cresol), are substantially volatilized out of the polymer sample after such an aging procedure. Consequently, the DTA temperature of such samples is drastically reduced by this treatment. On the other hand, the involatile antioxidants prepared according to the practice of this invention show very little change in DTA temperature after such a treatment.

Hence, the difference in DTA temperature before and after aging in argon affords a sensitive means of measuring volatility of an antioxidant. Furthermore, because the temperature is logarithmically related to antioxidant concentration, a small difference in temperature can reflect a large difference in antioxidant concentration as shown in the table below.

    ______________________________________                                         Relationship Between DTA Tem-                                                  perature and Concentration of                                                  4,4'-thiobis(6-t-butyl-m-cresol)                                                                Approximate DTA                                               Concentration %  Temperature ° C.                                       ______________________________________                                         .01              242                                                           0.1              259                                                           0.3              268                                                           0.5              273                                                           1.0              278                                                           2.0              282                                                           ______________________________________                                    

EXAMPLES 1-2

These examples illustrate formation of my polymeric antioxidants in situ. The antioxidant was formed by milling the bisphenol and peroxide into polyethylene and curing at 172° C. Both mixes contained 1 part per hundred of t-butyl perbenzoate which was equivalent to 1.04 mol per mol of bisphenol used. Comparative runs were made using the same formula except that the peroxide was omitted. The resulting compositions were compared by a differential thermal analysis comprising heating a reference and the antioxidant-containing sample at the same rate. Volatility of the antioxidant was indicated by comparing the DTA values before and after heat aging. The results are set forth in Table I.

                                      TABLE I                                      __________________________________________________________________________     Antioxidant      Color   DTA Performance ° C.                           __________________________________________________________________________                              With Peroxide                                                                             Without Peroxide                           __________________________________________________________________________                      Before                                                                             After                                                     Example                                                                             Type    p.h.r.                                                                             Cure                                                                               Cure                                                                               Unaged                                                                             Argon Aged                                                                            Unaged                                                                             Argon Aged                             __________________________________________________________________________     1    4,4'-butylidene-                                                                       2.1 White                                                                              White                                                                              261.2                                                                              252.9  263.5                                                                              232.7                                       bis(6-t-butyl-m-                                                               cresol)                                                                   2    4,4'-thiobis(6-t-                                                                      2.0 White                                                                              Light                                                                              285.7                                                                              284.1  281.6                                                                              266.1                                       butyl-m-cresol) Yellow                                                    __________________________________________________________________________

EXAMPLES 3-22

The method of Example 1 was repeated using various peroxides and 4,4' -thiobis(6-t-butyl-m-cresol). The results are set forth in Table II. It is seen that maximum retention of performance after aging in argon results when the weight ratio of peroxide to antioxidant precursor is between 1 and 2 to 1, the actual optimum value varying with the peroxide used.

                                      TABLE II                                     __________________________________________________________________________     Peroxide                  DTA Temperatures                                                       Cure Time at                                                 Example                                                                             Type     p.h.r.                                                                             340° F. (min.)                                                                  Unaged                                                                             Argon Aged                                       __________________________________________________________________________     3    No peroxide                                                                             --  10      281.6                                                                              260.3                                            4    Lupersol 101.sup.1                                                                      0.7 10      285.2                                                                              277.1                                            5    Lupersol 101                                                                             .8 10      285.4                                                                              281.3                                            6    Lupersol 101                                                                             .9 10      285.8                                                                              281.3                                            7    Lupersol 101                                                                            1.0 10      286.1                                                                              281.6                                            8    Lupersol 101                                                                            1.2 10      285.9                                                                              281.5                                            9    Lupersol 101                                                                            1.4 10      285.4                                                                              282.2                                            10   Lupersol 101                                                                            1.6 10      286.8                                                                              283.8                                            11   Lupersol 101                                                                            0.4 10      283.5                                                                              274.7                                            12   Lupersol 101                                                                            0.8 10      286.3                                                                              276.0                                            13   Lupersol 101                                                                            1.6 10      285.7                                                                              281.2                                            14   Lupersol 101                                                                            2.4 10      283.4                                                                              280.8                                            15   Lupersol 130.sup.2                                                                      0.4 20      283.6                                                                              263.4                                            16   Lupersol 130                                                                            0.8 20      284.3                                                                              278.4                                            17   Lupersol 130                                                                            1.2 20      284.8                                                                              281.0                                            18   Lupersol 130                                                                            1.6 20      285.3                                                                              284.1                                            19   Dicumylperoxide                                                                         1.3 10      285.6                                                                              283.6                                            20   Dicumylperoxide                                                                         1.5 10      285.3                                                                              283.0                                            21   Dicumylperoxide                                                                          1.75                                                                              10      285.0                                                                              283.5                                            22   Dicumylperoxide                                                                         2.0 10      284.0                                                                              284.3                                            __________________________________________________________________________      .sup.1 2,5-Dimethyl-2,5-bis(t-butyl peroxy)hexane                              .sup.2 2,5-Dimethyl-2,5-di(t-butyl peroxy)hexyne-3                       

EXAMPLES 23-26

These examples illustrate preparation of the polymeric antioxidant in a solvent. A mixture of 4,4'-thiobis(6-t-butyl-m-cresol) and di-t-butyl peroxide in xylene under nitrogen was heated in an autoclave and the product evaporated to dryness. The conditions are described in Table III. The molecular weight of the antioxidant polymer was determined by the freezing technique known as Beckman's method and set forth in Glasstone, Textbook of Physical Chemistry, 2 Ed (1947), 646-47. Molecular weight measurements in this experiment were made on 5% solutions in benzene.

The resulting polymeric antioxidant was incorporated into polyethylene and the DTA exotherm temperature was determined.

                                      TABLE III                                    __________________________________________________________________________                                           DTA Exotherm                                                                   Temperature ° C                   __________________________________________________________________________          Thiobisphenol                                                                         Concentration                                                                         Temperature                                                                           Time                                                                              Yield                                                                             Molecular                                      Example                                                                             kg.    gms/100 mls                                                                           ° C                                                                            Hrs.                                                                              %  Weight                                                                               Unaged                                                                             Argon Aged                           __________________________________________________________________________     23    0.108 14.5   155-160                                                                               2  105                                                                               876   287.5                                                                              283.2                                24   4.0    16.2   160    6  101                                                                               880   287.9                                                                              286.8                                25   15.0   35.8   160-167                                                                               3  101                                                                               1,287 283.5                                                                              285.1                                26    0.324 43.9   160-163                                                                               31/2                                                                              102                                                                               640   284.5                                                                              284.1                                __________________________________________________________________________

EXAMPLES 27-31

These examples illustrate the effect of pH on the polymer properties obtained using a ferricyanide to prepare the antioxidants. In Examples 27-31, the ferricyanide was dissolved in 80% of the water used and added dropwise over 3 hours to a mixture of 4,4'-thiobis(6-t-butyl-m-cresol), benzene, an acid acceptor, and the remaining water. Molecular weights of the product were determined according to the method of Example 23.

                                      TABLE IV                                     __________________________________________________________________________          Thiobisphenol                                                                         K.sub.3 Fe(CN).sub.6                                                                           Water                                                                              Benzene                                                                             Molecular                                 Example                                                                             g.     g.    Type pH g.                                                                               mls.                                                                               mls. Weight                                                                               Appearance                          __________________________________________________________________________     27   100     90   NaOH 7-11                                                                              20                                                                               300 200  663   Light green                         28   120    146   NaOH 7-11                                                                              30                                                                               400 200  860   Green brown                         29   120    146   Na.sub.2 CO.sub.3                                                                   8-10                                                                              60                                                                               500 200  890   Deep yellow                         30   120    146   NaHCO.sub.3                                                                         6-8                                                                               42                                                                               500 200  900   Light yellow                        31    120*  146   NaHCO.sub.3                                                                         6-8                                                                               42                                                                               500 200  944   Very light yellow                   __________________________________________________________________________      *The thiobisphenol used in this example had been recrystallized from           benzene-cyclohexane.                                                     

EXAMPLES 32-39

The following examples illustrate the effect of varying reaction conditions on the molecular weight, color and DTA performance of the antioxidant. The reaction conditions set forth in Example 27 were followed except that in Examples 37, 38 and 39, the ferricyanide solution was added in one hatch at the beginning of the reaction. All experiments used analytical reagent grade benzene except 39 which used a technical grade. Experiments 38 and 39 used technical grade ferricyanide while the others used analytical reagent grade ferricyanide. The molar ratio of ferricyanide to thiobisphenol in the above experiments is uniformly about 1.33 to 1. The results are set forth below in Table V.

                                      TABLE V                                      __________________________________________________________________________                                                      D.T.A.                             Thiobisphenol                                                                         Ferricyanide                                                                          NaHCO.sub.3                                                                         Benzene                                                                             Water                                                                              Temp.     Molecular                                                                            Performance                   Example                                                                             g.     g.     g.   ml.  ml. ° C                                                                         Color Weight                                                                               Unaged                        __________________________________________________________________________                                                      ° C.                   32   300    365    105  650  1250                                                                               15-30                                                                              Lt. Yellow                                                                           922   289.0                         33   300    365    105  650  1250                                                                               50  Yellow                                                                               977   289.4                         34   500    608    175  1080 2218                                                                               35  Lt. Yellow                                                                           937   288.6                         35   1250   1520   437  2100 5000                                                                               35  Lt. Yellow                                                                           1000  288.5                         36   300    365    105  650  1000                                                                               25  Lt. Yellow                                                                           955   289.8                         37   1500   1824   520  2500 4500                                                                               35-50                                                                              Lt. Yellow  289.3                         38   300    365    105  650  1000                                                                               35  Lt. Yellow                                                                           924                                 39   300    365    105  650  1000                                                                               35  Lt. Yellow                                __________________________________________________________________________

EXAMPLES 40-45

Polymeric antioxidants made according to the present invention were incorporated into polyethylene at various levels irradiated to 10 megarads, except the 2 p.h.r. level which was given a dose of 20 megarads. The irradiated products were evaluated for heat aging characteristics and modulus of elasticity at a variety of temperatures. The results are shown in Table VI and Table VII. The compositions in Table VI and Table VII were prepared in slab form, except for the 2 p.h.r. values which were prepared in the form of 0.125 in i.d., 0.020 in thick tubing.

The time to crack on flexing was determined by heating the samples to the indicated temperature for the indicated time and bending them through 180° with cracking observed visually. The values in Table VI represent the average of the times to slight cracking and the times to serious cracking.

The elastic modulus was determined above the melting point of the polymer by statically loading strips of polymer and measuring the strain. The initial slope of the resulting stress-strain curve was taken as the elastic modulus.

These examples illustrate the marked advantage of polymeric antioxidants at high concentrations and elevated temperatures. At lower concentration, oxidation occurs so rapidly that the effect of volatilization is masked. At lower temperatures, volatilization is not a problem.

                                      TABLE VI                                     __________________________________________________________________________     Time to Crack on Flexing at 155° C and 175° C                    Antioxidant              Concentration (p.h.r.)                                             0.063 0.125 0.25  0.5   1.0    2.0                                __________________________________________________________________________     Example                                                                             Type    155°                                                                       175°                                                                       155°                                                                       175°                                                                       155°                                                                       175°                                                                       155°                                                                       175°                                                                       155°                                                                        175°                                                                       200°                        __________________________________________________________________________     40   4,4'-thiobis(6-t-                                                                      71 29 144                                                                               41 271                                                                               79 623                                                                               172                                                                               >600*                                                                              320                                                                               78                                      butyl-m-cresol)                                                           41   A       71 30 154                                                                               48 293                                                                               94 752                                                                               220                                                                               >600*                                                                              490                                                                               130                                42   B       69 29 160                                                                               50 374                                                                               86 793                                                                               217                                                                               >600*                                                                              580                                                                               162                                __________________________________________________________________________      NOTE: A was prepared from 4,4'-thiobis(6-t-butyl-m-cresol) using               di-t-butyl peroxide as shown in Example 23 while B was prepared using          potassium ferricyanide as in Example 35.                                       *These tests were abandoned after all samples at the 1% level had              withstood 600 hours at 155° C without cracking.                   

                  TABLE VII                                                        ______________________________________                                         Modulus (p.s.i.) at 160° C.                                             ______________________________________                                         Antioxidant             Concentration (p.h.r.)                                 Ex-                                                                            ample Type        0.0   0.063 0.125 0.25 0.5 1.0 2.0                           ______________________________________                                         43    4,4'-thiobis(6-t-                                                                          30    --    10    10    7  7   21                                  butyl-m-cresol)                                                          44    A           --    --    33    19   19  9   56                            45    B           --    --    21    15   17  9   69                            ______________________________________                                    

EXAMPLES 46-51

A further series of thiobisphenolic antioxidants were prepared as shown in Table VIII, following the procedure of Examples 27-31, supra. Molecular weight values reported are number average molecular weights determined by vapor phase osmometry.

                                      TABLE VIII                                   __________________________________________________________________________     Antioxidants Based on 4,4'-thiobis-                                            (6-t-butyl-3-methylphenol)                                                     __________________________________________________________________________          Potassium                                                                             4,4'-thiobis-                                                                               Sodium                                                                               Molecular                                       Example                                                                             Ferricyanide                                                                          (6-t-butyl-3-methylphenol)                                                                  Bicarbonate                                                                          Wt.                                                  (g.)   (g.)         (g.)                                                  __________________________________________________________________________     46    906    660          246  1697                                            47    970    660         263   2393                                            48    810    660         220   1076                                            49a  2040   1500         553   1360                                            49b  2040   1500         553   1490                                            50   1930   1500         524   1189                                            51a  2170   1500         590   1450                                            51b  2170   1500         590   1869                                            __________________________________________________________________________

The duplicate preparations 49a and 49b were combined together for testing (average molecular weight 1430) and labelled 49. Similarly 51a and 51b were combined, (M.W. 1660) and labelled 51.

The antioxidants shown in Table VIII were blended into a low density polyethylene (DFD 6040, Union Carbide Corp.) irradiated to various dose levels and the elastic modulus (Young's modulus) at 160° C measured. The samples irradiated to 20 mrads dose were hung in a circulating air oven at 185° C, and their room temperature elongation measured after various exposure times. One sample containing tetrakis [methylene 3-(3',5'-di-t-butyl-4'-hydroxyphenyl) propionate] methane (Irganox 1010) was included in these tests for comparison purposes. All antioxidants were incorporated into DFD 6040 (0.2 M.I.; 0.92 density) at the 2% w/w level.

                                      TABLE IX                                     __________________________________________________________________________                   Elastic Modulus                                                                         Elongation at room temperature                                        at 160° C after                                                                  after days at 185° C                             Antioxidant                                                                           Mol. Wt.                                                                            10                                                                               20                                                                               40                                                                               60 mrads                                                                            0  8  14 17 21                                          __________________________________________________________________________     50     1190 5 13                                                                               40                                                                               63   560                                                                               290                                                                               230                                                                               105                                                                               60                                          49     1430 6 16                                                                               44                                                                               77   550                                                                               340                                                                               190                                                                               125                                                                               65                                          51     1660 5 16                                                                               47                                                                               81   540                                                                               360                                                                               180                                                                               110                                                                               30                                          47     2390 6 18                                                                               51                                                                               86   550                                                                               290                                                                               170                                                                               105                                                                               25                                          Irganox 1010                                                                          --   7 18                                                                               48                                                                               73   520                                                                               <20                                                                               <20                                                                               -- --                                          __________________________________________________________________________

EXAMPLES 52- 57

Other oligomeric antioxidants were prepared from 4,4'-butylidene-bis(6-t-butyl-3-methylphenol) as in the previous example. Table III shows the recipes used in the preparation and the molecular weights of the products, determined as in Examples 46- 51. Preparations were based on 1500 g. of the antioxidant starting material. 4700 ml. of dearated water was used in each reaction to dissolve the salts.

                  TABLE X                                                          ______________________________________                                               Potassium   Sodium                                                       Sample                                                                               Ferricyanide                                                                               Bicarbonate        Molecular                                 No.   (g.)        (g.)       Benzene Wt.                                       ______________________________________                                         52    1840        500        5400    1230                                      53    2040        553        5400    1790                                      54    1940        500        5400    1460                                      55    1840        500        5400    1240                                      56    1650        500        6500    1340                                      57    1750        500        6500    1610                                      ______________________________________                                    

Certain of the foregoing antioxidants were milled into polypropylene (Profax 6823; flow rate (g./10 min.), 0.4 (ASTM-D1238L); 0.903 density) and aged at 150° C in a air circulation oven. The results are given in Table XI. Some of these materials were also blended with distearyl pentaerylthritol disphosphite and blended into polyethylene containing a polyfunctional monomer (triallyl isocyanurate). The irradiated samples were aged at 175° C and the results are shown in Table XII.

                                      TABLE XI                                     __________________________________________________________________________              1   2    3   4     5   6    7    8                                    __________________________________________________________________________     Profax 6823                                                                             99.7                                                                               99.7 99.7                                                                               99.7  99.7                                                                               99.7 99.7 99.7                                 Irganox 1010                                                                             0.3                                                                               0.075                                                                               --  --    --  --   --   --                                   Dilauryl Thio-                                                                 dipropionate                                                                            --  0.225                                                                               --  --    --  0.225                                                                               0.225                                                                               0.225                                Sample 52                                                                               --  --    0.3                                                                               --    --  0.075                                                                               --   --                                   Sample 53                                                                               --  --   --   0.3  --  --   0.075                                                                               --                                   Sample 55                                                                               --  --   --  --     0.3                                                                               --   --   0.075                                         Average ultimate elongation (at 2"/min.) following heat aging at               150° C, %                                                      Original              500-530%                                                 4 weeks  267 355  450 320   210 --   210  --                                   5 weeks  225 333  367 330   --  450  170  --                                   6 weeks  180 320  350 --    --  390  --   290                                  7 weeks  --  250  --  --    --  243  100  300                                  __________________________________________________________________________

                  TABLE XII                                                        ______________________________________                                         Samples irradiated to 25 mrads                                                              1      2      3       4    5                                      ______________________________________                                         DFD 6040       95       95     95    95   95                                   Triallyl Isocyanurate                                                                          3        3      3     3    3                                   Irganox 1010    2       --     --    --   --                                   Distearyl pentaerythritol                                                      diphosphite    --        1      1     1    1                                   Sample 52      --       --      1    --   --                                   Sample 53      --       --     --    1    --                                   Sample 55      --       --     --    --    1                                   M100 at 160° C, psi                                                                    91       36.5   83    85   86                                   Average elongation (at 2"/min.) after heat aging (at 175° C, %          Original                       >500%                                           1 day          310      447    333   253  310                                  2 days         277      420    320   177  282                                  4 days         140-280  367    283   <20  270                                  7 days         <20      <20    30-300                                                                               <20  293                                  ______________________________________                                    

The above examples are not to be construed as limiting but merely illustrative of the scope of the present invention. Thus, it can be seen that the antioxidants of the present invention can be formed from a wide variety of bisphenol materials. The antioxidants are generally effective in all polymers, particularly those which are known to be stabilized by hindered phenols, and they can be incorporated in such polymers by conventional methods such as hot milling, mixing in a Banbury mixer, extrusion, etc. 

I claim:
 1. An antioxidant composition comprising a mixture of bisphenolic polymers of formula ##EQU1## wherein n is an integer of from 2 to about 12 and wherein, Y is independently chosen from the group consisting of sulfur, methylene and butylidene, and for each bisphenolic repeating unit --Z - Y - X-- , Z and X are independently selected from the group consisting of ##SPC7##R₂ and R₃ being independently selected from the group consisting of hydrogen and alkyl, cycloalkyl, aryl and alkaryl groups having from 1 to 8 carbon atoms, R₄ being an alkyl group of from 4 to 8 carbon atoms.
 2. An antioxidant polymer according to claim 1 wherein each R₄ is a tertiary alkyl group.
 3. An antioxidant polymer according to claim 1 wherein each Z and X moiety are ##SPC8##
 4. An antioxidant polymer according to claim 3 wherein R₄ is a tertiary alkyl group.
 5. An antioxidant according to claim 1 wherein R₂ is methyl, R₃ is hydrogen, and R₄ is t-butyl.
 6. An antioxidant according to claim 3 wherein R₂ is methyl, R₃ is hydrogen, and R₄ is t-butyl.
 7. An antioxidant polymer according to claim 1 wherein Y is sulfur.
 8. An antioxidant polymer according to claim 1 wherein Y is methylene.
 9. An antioxidant polymer according to claim 1 wherein Y is butylidene.
 10. An antioxidant composition of bisphenolic polymers of structure ##STR2## wherein average n for said composition is from about 2 to about 4 an wherein Y is chosen from the group consisting of sulfur, methylene and butylidene independently, and for each bisphenolic repeating unit -- Z - Y - X--, Z and X are independently selected from the group consisting of ##SPC9## R₂ and R₃ being independently selected from the group consisting of hydrogen and alkyl, cycloalkyl, aryl or alkaryl groups having from 1 to 8 carbon atoms, R₄ being an alkyl group of from 4 to 8 carbon atoms.
 11. A composition according to claim 10 wherein R₄ is a tertiary alkyl group.
 12. A composition according to claim 10 wherein each Z and X moiety is ##SPC10##
 13. A composition according to claim 10 wherein R₂ is methyl, R₃ is hydrogen, and R₄ is t-butyl.
 14. A composition according to claim 10 wherein R₄ is a tertiary alkyl group, and Y is sulfur or butylidene.
 15. A composition according to claim 10 wherein R₂ is methyl, R₃ is hydrogen, and R₄ is t-butyl, and Y is sulfur or butylidene. 